This invention relates in general to breath alcohol measuring instruments and in particular to a new and useful calibrating device for such instruments.
Measuring instruments with semiconductor sensors in which the electrical conductivity of the sensor material is influenced by the adsorption of gas need a possibility for calibration because the semiconductor sensor signal varies due to use and aging. Depending on performance and accuracy required, calibration may also be necessary in shorter time intervals.
One known calibrating device for equipment for the automatic analysis of traces of organic solvent vapors in air, using a semiconductor sensor, consists of a thermo-insulated and heated tank like a washing bottle filled to a part of its height with liquid solvent. Disposed in the liquid and projecting into the gas-filled space of the tank is an evaporating device comprising a filter paper folded in star form. An inlet tube for the air ends above the liquid level. The outlet for the discharge of the solvent/vapor concentration from the calibrating device is located in an upper dome of the tank. The outlet empties via a capillary and a control valve in the test gas line. The test gas line is connected to the semiconductor sensor, which is followed by a flow regulator and the gas pump. A program switching mechanism takes care of the switching between measuring and calibrating in fixed time intervals. During the measuring phase the air to be tested is sucked by the pump through the test gas line to the semiconductor sensor. During the calibrating phase a filter is inserted upstream of the test gas line to rid the air of the measuring component. Then the underpressure prevailing in the test gas line attracts, through the capillary and the open control valve, solvent vapor from the calibrating device, which mixes with the test gas line air free of the measuring component, and feeds it to the semiconductor sensor. To prevent condensation from the solvent vapor then standing in the outlet upon the conclusion of the calibration, a bypass line, provided with a capillary, connects the outlet constantly to the flow regulator and the pump while bypassing the control valve and the semiconductor sensor. Thus, a part flow is taken from the calibrating device constantly to replenish the content of the outlet. It is disadvantageous that the device is position-dependent due to the use of liquid. The concentration formed in the calibrating device depends greatly on the temperature which must be kept constant by costly insulation and additional heating with high energy requirement. In addition to previous condensations, the concentration contained in the test gas line is also affected by the flow and pressure conditions at the mixing point and impaired in its reliability (DE-PS 24 22 271).
In a known device for the detection and quantitive analysis of a gas, a sample, such as a breath sample to be tested for alcohol, is blown into a sample collector. The sample collector is a wound, narrow tube with a thermal jacket, open to environment at one end, and alternately connected at the other end to the blow-in opening, the electrochemical sensor, or a calibrating device. For measuring, the gas is sucked by a pump from the sample collector through the sensor and a flow meter and measured by the sensor. Each measurement is preceded by a calibration. For this purpose, the sensor is connected first to the blow-in opening, and ambient air is attracted by the pump for flushing. At the same time, the sample collector is connected to the calibrating device, a compressed gas cylinder containing air and ethanol vapor. The composition is selected so that no ethanol condenses at the lowest operating temperature, and that no water vapor is present either. When an actuating button is depressed, calibrating gas flows from the compressed gas cylinder into the sample collector, filling it. This calibration sample is subsequently fed to the sensor and the latter's display corrected according to the known concentration. Disadvantageous is that no means are provided to determine the amount of calibrating gas filled into the sample collector. If the actuating button is pushed too briefly, the amount is too small, and the calibration leads to faulty results. When pushed too long, the sample collector becomes over-filled, and calibrating gas discharges into the atmosphere at its open end and is lost. Thus, the calibrating gas supply is exhausted prematurely (DE-OS 22 40 422).